The present invention relates to borehole logging and particularly to a method for detecting and locating the presence of fractures in a formation penetrated by a borehole. Various acoustic logging systems have been developed in an attempt to detect the presence of fractures in a formation penetrated by a borehole but these methods are not entirely successful. The presence of fractures in a formation can be very important in the petroleum industry in deciding whether a formation is capable of commercial production of hydrocarbons. Fractured formations are difficult to detect since they do not provide a consistent, uniform, or recognizable response in any available electrical, nuclear or acoustic logging device or method that is not routinely produced by conditions or effects not related to the presence of fractures.
One method that has been proposed for detecting fractures is an acoustical mapping method such as described in U.S. Pat. No. 3,369,626. In this patent, an ultrasonic source is triggered to direct a beam of ultrasonic energy radially out to impinge upon the borehole wall and the amplitude of the reflected signal is recorded. The recording is normally done on a cathode ray oscilloscope which is photographed to provide a permanent record of the borehole wall. This method is successful in locating fractures under those conditions in which the fracture is open an amount sufficient to reduce the amplitude of the reflected signals. Another method that has been devised for locating vertical fractures in a formation is that described in U.S. Pat. No. 4,130,816. This patent describes a logging tool in which a transducer admits a burst of sonic energy that is so directed that it produces shear waves in the formation. The shear waves travel around the circumference of the borehole and are received at a receiving transducer. The amplitude of the received shear waves is then recorded to determine the locations of fractures since an open fracture will reduce the amplitude of the received shear wave.
In addition to the above acoustic methods it has also been suggested that a moldable plastic or soft rubber could be pressed against the borehole wall to obtain an impression of the borehole wall that would indicate the presence of fractures. It is obvious that this type of device will only work where the fractures are open and of a substantial width. Since many fractures are small, this method is of limited usefulness in locating formations capable of commercial production.